OVER JUST A little more than a decade, the crypto world has exploded from a single currency to millions of coins and assets, each promising a small share in the next big thing. The challenge for anyone putting their money into that minefield-posing-as-a-goldmine is to distinguish digital treasure from the many, many scam-ridden penny stocks of the digital economy. A new study has put a number to just how prevalent those garbage assets have become: About a quarter of the new crypto tokens launched last year—counting only those that gained any value at all—were clear-cut, short-term cons, scamming buyers within a week of their release.